Oil wells are often produced by mechanical pumps when reservoir pressure cannot force oil to the surface. A common method for producing a "pumping" well includes the use of a reciprocating "polished" rod extending up through a stuffing box at the wellhead, and connected at its lower end to a "sucker" rod string extending down to a downhole pump in the well. The polished rod and sucker rod string are reciprocated vertically by a conventional crank mechanism at the surface, and the downhole pump lifts oil to the surface through a well tubing or casing, which usually extends from the producing formation to the wellhead.
The stuffing box includes packing compressed to make a sliding seal against the polished rod to prevent leakage of fluid, such as oil or gas, from the wellhead.
Although the polished rod is highly polished to minimize friction and wearing of the packing, from time to time the packing wears sufficiently to permit fluid to leak past it and into the atmosphere. Such leaks are wasteful and adversely affect the environment.
The typical prior art stuffing box has a conventional packing gland and nut, which can be manually adjusted by field personnel at the wellhead in an effort to control leaks. Manual adjustment by field personnel is subject to human error and variation, often resulting in improper force being applied to the packing material in the stuffing box. If the force is too great, the packing wears prematurely and increases the power required to drive the pump. If the packing is too loose, the stuffing box leaks.
In addition to packing wear, a polished rod occasionally breaks, resulting in the upper portion of the broken rod being pulled up from the stuffing box by the crank. The lower portion of the broken rod and the sucker rod string drop down to the bottom of the pump stroke, resulting in the lower portion of the broken rod dropping clear of the packing in the stuffing box. To avoid loss of reservoir fluids, and to minimize damage to the environment, many stuffing boxes include a flap valve intended to snap shut and close in the well when the polished rod breaks.
Some prior art flap valves include a spring which urges the valve flap toward a closed position, and against the polished rod while the pump is operating. This causes the valve flap to drag along the polished rod surface as the rod reciprocates, resulting in wear on the flap, or damage to the rod, or both. Other flap valves include complicated mechanisms for actuating the flap to close the valve when the polished rod breaks. In another type of valve, the flap is mounted to hang free of the polished rod, and rely only on fluid flow from the well to urge it closed if the polished rod breaks. The latter arrangement avoids complicated mechanisms and undesirable wear of the valve or polished rod, but sometimes is slow-acting, or fails to act at all.